A new review assesses evidence that “transcranial direct-current stimulation” can alter physical limits.

One of the oddest topics I’ve covered over the past few years is the use of electric brain stimulation to enhance endurance. Back in 2013, I wrote about a Brazilian study that used a technique called transcranial direct-current stimulation, or tDCS, to reduce the perceived effort and boost peak power output in a group of national-class cyclists—a result that seemed to confirm the long-suspected claim that the limits of endurance lie in your head, not your muscles.

I expected that the result would spark an explosion of research into the athletic uses of brain stimulation, but these things take time. Certainly, there has been some interest from the sports world. In 2014, I wrote about a Red Bull research project that subjected four elite cyclists and triathletes to tDCS; last year, I wrote about the Golden State Warriors’ use of souped-up headphones from a company called Halo Neuroscience that are designed to deliver tDCS during their pre-workout warm-up.

Actual research, though, has been slower to appear. So I was interested to see a new review that appeared last month in Frontiers in Physiology (full text freely available here), from researchers at the University of Kent’s Endurance Research Group led by Alexis Mauger, assessing the evidence for tDCS to improve exercise performance.

tDCS basically involves sticking two electrodes to your head and connecting them to a voltage source (a 9-volt battery, say) to run a very weak current through your brain. The current is too weak to make your neurons fire directly, but enough to “prime” their excitability so that they become either more or less likely to fire, depending on the direction of the current.

The precise effects depend on where you place the electrodes and which regions of the brain get the current, but researchers have published thousands of studies in recent years looking at effects like enhanced learning and improved motor control in conditions like Parkinson’s disease. The hype around tDCS has certainly outstripped the evidence, but it’s pretty clear that the technique does something.

Mauger and his colleagues identified 12 different studies—a surprisingly large number, mostly published since 2015—that have assessed tDCS and exercise performance, using methodologies such as cycling or isolated muscle contractions. All are small (the biggest had 18 subjects), which puts some serious limits on the conclusions we can extract. Still, in eight of the studies, tDCS triggered improved performance compared to sham stimulation.

The studies encompass a variety of parameters, including stimulation time (10 to 20 minutes), current (1.5 to 2.0 mA), and electrode placement. Most of the studies aim to simulate the motor cortex, under the theory that exciting the neurons in the motor cortex might counteract the decline in brain-to-muscle signal that occurs during fatigue. Others, including the Brazilian study that I wrote about in 2013, stimulate areas associated with perceptions of pain and effort.

The protocols are too varied to draw any general conclusions—but, as the authors point out, the results are encouraging enough to stimulate further research and hopefully develop some standardized protocols.

There’s a really important caveat, of course. Do we really want to usher in an era of “brain doping”? Frankly, I’d like to see anti-doping authorities get ahead of the curve and weigh in right now to restrict the use of electric brain stimulation for athletic enhancement. It’s a restriction that would be very difficult to police, since there’s currently no way to test for recent use of tDCS—but simply having the rule in place would act as a deterrent to some degree.

That said, I think this technique has the potential to radically advance our understanding of the brain’s role in endurance. So far, to paraphrase a friend of Mark Twain’s, everybody talks about how important the brain is, but nobody does anything about it. With tDCS and related techniques, we have a chance to figure out how different parts of the brain influence our perceived limits—and, perhaps, how much we keep in reserve.